Simple Packet Relay Transport (SPRT) is a reliable transport protocol designed for IP networks. The SPRT protocol uses a windowing mechanism for flow control. The protocol establishes a transmit window size which requires an allocation of a corresponding transmit buffer space. The transmitter sends packets which are received into the receive buffer. Receipt of the packets is acknowledged to the sender. As the packets are retrieved from the receive buffer, they are consumed by the recipient and the consumption of packets is acknowledged to the sender.
The SPRT protocol restricts the size of the transmit window, preventing the transmitter from sending more than the number of packets allocated to the window size (Nwnd) without receipt of consumption acknowledgment from the receiver. The transmit buffer will fill to the size of the transmit window and send packets to the network at the appropriate network rate. The transmitter continues to send packets from the buffer and continues to refill the buffer as long as a consumption acknowledgment is received. Once the transmitter sends a number of packets equal to Nwnd without receipt of a consumption acknowledgment, the transmit buffer will refill but will not transmit until a consumption acknowledgment is received. If the transmit window is too small, the IP link will have idle periods where no packets are transmitted after the transmit window is emptied, while the transmitter is waiting for a consumption acknowledgment. The transmitter must wait for receipt of an acknowledgment before it can send additional packets.
In order to fully utilize the IP capacity, without idle transmit times, the transmit window must be large enough to continue transmitting until consumption acknowledgment is received. If the transmit window is exhausted before a consumption acknowledgment is received, then transmission will stop, waiting for a consumption acknowledgment, thus wasting bandwidth.
Because an undersized window will waste bandwidth, large windows are more often implemented in order to avoid wasted bandwidth. However, a large window will consume additional memory space in the sender. A window which is larger than appropriate for the network and data rate, will provide no performance benefits. Because memory requirement for buffering transmission packets are substantial, it is desirable to limit the window size to the smallest possible value without losing performance.